This invention relates generally to transistor switching circuitry, and more particularly the invention relates to push-pull transistor circuitry.
In a push-pull circuit, an output terminal is alternately switched between positive and negative (or ground) voltage potentials. Such circuits can be used in a voltage converter such as the Linear Technology LT1054. In the LT1054 device the transistors are designed to handle 300-400 milliamperes (mA) of current. Since they are connected serially between positive and ground potentials, if the two transistors in the LT1054 are allowed to conduct simultaneously, large supply currents will flow, wasting power and possibly damaging the circuit.
The transistors are driven into saturation when they are conducting, and this results in excess charge in the base region of the conducting transistor. The base charge slows the turn-off time for the transistor, since the excess charge must be removed or allowed to recombine before the transistor turns off. In turning off a saturated transistor, there is an initial drop of about 10% in the base/emitter voltage. After the initial drop, the base/emitter voltage stays high until all the excess charge is either removed or recombines with majority carriers. During this period, the transistor is still saturated or conducting large currents. Once the excess charge is removed, the base/emitter voltage collapses and the transistor is turned off. Thus, as long as the transistor base/emitter voltage is significant, the transistor is considered turned on, and the fact that the base/emitter voltage collapses is an indication of transistor turn-off.